Pathophysiology of bronchiolitis vs. bronchitis

Question

In most medical textbooks, bronchitis is said to be an inflammation of both bronchi and bronchioles, and to affect mostly adults.

On the other hand, bronchiolitis is said to be a specific inflammation of bronchioles, and to affect almost exclusively infants.

There must be an explanation for this epidemiological difference, but I am having a hard time finding one. For example, the MSD Manual has a section about the pathophysiology of bronchiolitis:

The virus spreads from the upper respiratory tract to the medium and small bronchi and bronchioles, causing epithelial necrosis and initiating an inflammatory response. The developing edema and exudate result in partial obstruction, which is most pronounced on expiration and leads to alveolar air trapping. Complete obstruction and absorption of the trapped air may lead to multiple areas of atelectasis, which can be exacerbated by breathing high inspired oxygen concentrations.

... But it does not explain why the mechanisms involved would not apply to adults.

On a pathophysiological level, how can one explain that bronchiolitis targets infants, whereas bronchitis targets adults?

Answer 1

The reason the distinction if confusing, is because while both terms appear to be referring to inflammation of a particular anatomical / histologic structure, in reality, most clinicians use "bronchiolitis" to refer to a clinical syndrome.

Here is an excerpt from UpToDate (sorry possible pay wall):

Bronchiolitis is broadly defined as a clinical syndrome of respiratory distress that occurs in children <2 years of age and is characterized by upper respiratory symptoms (eg, rhinorrhea) followed by lower respiratory (eg, small airway/bronchiole) infection with inflammation, which results in wheezing and or crackles (rales). Bronchiolitis typically occurs with primary infection or reinfection with a viral pathogen, but occasionally is caused by bacteria (eg, Mycoplasma pneumoniae). In young children, the clinical syndrome of bronchiolitis may overlap with recurrent virus-induced wheezing and acute viral-triggered asthma.

Adults also experience bronchiolitis (the inflammation), but it is most often due to non-infectious etiologies. As Ryu and colleagues note (2020. PMCID 7281671):

Respiratory bronchiolitis (RB) is likely the most common form of bronchiolitis and is usually related to cigarette smoking.

Other non-infectious etiologies include constrictive (obliterative) bronchiolitis, follicular bronchiolitis, and diffuse aspiration bronchiolitis (Ryu et al 2020. PMCI 7281671).

Even still, some adults also experience infectious acute bronchiolitis (akin to the pediatric syndrome). A review of 20 adult patients with infectious bronchiolitis by Ryu and colleagues in a different paper revealed mostly bacterial pathogens (2015. PMID 26524622):

Detected organisms included Mycoplasma pneumoniae in eight (40.0%) patients, influenza virus in two (10.0%), influenza virus and Streptococcus pneumoniae in two (10.0%), Haemophilus influenzae in three (15.0%), and respiratory syncytial virus and rhinovirus in one (5.0%) patient.

Some authors have suggested that it is the relatively more mature innate immune system that is the underlying reason adults rarely experience primarily bronchiolar symptoms (Lambert and Culley 2017. PMCID 5694434).

^{Figure 1 from Lambert and Culley available here.}

Innate immunity to infection in the lung in early life. Alveolar macrophages (AM) are the most numerous leukocyte in the lungs in early life. Reduced cytokine production and phagocytic ability in AM in early life compared to those of adults could underlie susceptibility to infection